As a camera for imaging a wide angle field, a fish-eye camera is available. The fish-eye camera can take an image of a hemispherical area around an optical axis by using a plurality of lenses having a high curvature.
As a wide-angle camera device using a reflection surface, a so-called omnidirectional camera disclosed in Japanese Patent No. 2939087 is available. With a reflection surface having an axial rotary symmetrical shape around the optical axis of the camera, an image of 360 degrees around the rotation axis can be formed on an imaging surface at a time.
A fish-eye camera has a special optical system for forming a fish-eye image, and the lens has a complicated shape and is expensive. Further, since an image taken by a fish-eye camera is different in distortion degree between the center and the periphery, the center and the periphery of the image taken by the fish-eye camera are quite different in distortion amount, resulting in a strange image.
Meanwhile, the omnidirectional camera using a reflection surface can take an image of 360 degrees at a time and can generate a panoramic image by image conversion.
Although such an omnidirectional camera is useful for a monitor area of 360 degrees, in the case of a monitor area of about 180 degrees, the omnidirectional camera has a number of unused pixels in an imaging element, resulting in inefficiency.
To be specific, since the omnidirectional camera takes an image of 360 degrees as a circular image, when an image is formed on a rectangular imaging surface (e.g., CCD), the image needs to be formed as a circle whose diameter is a short side of the rectangle, resulting in a number of unused pixels. For example, when it is assumed that an image is formed on a CCD surface having an aspect ratio of 4:3, a range usable as an image is just 59% of the entire CCD area because the image is a circle having the short side (:3) as a diameter, which is inefficient.
Moreover, since an image reflected once is imaged, the image is flipped and thus it is difficult to directly recognize the state from the taken image.